Various schemes have heretofore been proposed for heat control of plastic resin being converted in apparatus from solid into melted form. Thus, for example, U.S. Pat. No. 2,721,729 issued Oct. 25, 1955 to J. W. Van Riper discloses a plastic extruder in which plastic material is advanced through a barrel by a screw so as to be extrudable from a delivery head at the front of the barrel. In the Van Riper extruder, the barrel has formed therein a plurality of deep circumferential grooves spaced axially over substantially the full length of the barrel and used both as seats for heating bands for heating the barrel and as recesses for receiving cooling fluid for cooling the barrel. Such heating and cooling arrangement for the Van Riper apparatus has, however, the disadvantages that the grooves employed therein are costly to fabricate, the arrangement is wasteful because it provides for liquid cooling of the rear part of the barrel where such cooling is not needed, and the arrangement lacks features which structurally differentiate the arrangement over the length of the barrel such that the arrangement is structurally adapted to retain more heat over the barrel's rear part than is retained over its first part, and to lose more heat over the barrel's front part than is lost over the rear part.